After many false alarms in recent years, NASA confirmed in September that Voyager 1 has become the first human-made object to reach interstellar space.

They've got three pieces of evidence to prove it. The first two were pretty straightforward: A drop in solar wind and a bump in galactic cosmic rays. These changes are to be expected when an object leaves the area of the sun's influence.

But the third bit of evidence — the one needed to say for sure that the spacecraft was in uncharted territory — was only made possible by a serendipitously timed explosion on the sun.

As charged particles released by the explosion reached Voyager 1, one of its instruments was able to measure the density of the plasma surrounding the spacecraft — and that data confirmed that the spacecraft had gone interstellar.